[0001] The invention relates to a radio-frequency identification, RFID, transponder, in
particular to an active load modulation, ALM, RFID transponder and to a method for
sending an RFID message from an RFID transponder to a reader.
[0002] RFID technology is for example used for communication between an RFID transponder
and a reader, in particular an RFID reading device. For example magnetic coupling
between an antenna of the RFID transponder and an antenna of the reader is established.
Communication is performed for example by means of radio-frequency, RF, fields with
a frequency in the order of MHz, for example at 13.56 MHz. ALM RFID transponders communicate
to the reader by generating a transmission signal to modulate a reader antenna signal
on a reader antenna.
[0003] In order to reduce space consumption, RFID transponders with very small dimensions
and consequently very small antenna sizes are required. A drawback of such small transponder
antennas is a reduction of a possible operating range due to a limited load modulation
amplitude at the reader antenna. Consequently, a reduction of the antenna size is
limited in existing transponders.
[0004] In some existing approaches, transmission from the RFID transponder to the reader
may be active only during times corresponding to modulation periods in passive load
modulation devices. Such approaches may suffer from a reduced load modulation amplitude.
In other approaches, the transmission may be active also during times corresponding
to non-modulation periods in passive load modulation devices to increase the load
modulation amplitude. A drawback of such approaches may be that more than two amplitude
levels of the reader antenna signal may be caused. Therefore, the reader may have
difficulties to correctly demodulate the reader antenna signal, which may lead to
an increased error rate in the communication.
[0005] Furthermore, in existing approaches, an absolute value of a change, in particular
an amplitude change, of the reader antenna signal at the beginning of a transmission
frame may be different from an absolute value of the amplitude change of the reader
antenna signal caused by modulation during the transmission frame. This may lead to
an increased error rate for detecting a start of a message or to a reduced success
rate of transponder reply detection.
[0006] It is therefore an object to provide an improved concept for RFID communication reducing
errors in communication and/or the detection of the start of a message, while allowing
for an increased operating range at the same time.
[0007] This object is achieved by the subject matter of the independent claims. Further
implementations and embodiments are subject matter of the dependent claims.
[0008] According to the improved concept, ALM is used for sending a message, in particular
an RFID message, from an RFID transponder to a reader during a transmission frame.
An encoded bit signal changes during the transmission frame between a first logic
level and a second logic level, wherein the encoded bit signal has the first logic
level at the beginning of the transmission frame. A transmission signal is generated
during the transmission frame having a first phase when the encoded bit signal has
the first logic level and having a second phase when the encoded bit signal has the
second logic level. During a time interval preceding the transmission frame, the transmission
signal is generated having the second phase.
[0009] In this way, it may be achieved that an amplitude value of a reader antenna signal
on an antenna of the reader during the time interval preceding the transmission frame
is the same as when the encoded bit signal has the second logic level during the transmission
frame.
[0010] According to the improved concept, an RFID transponder is provided. The RFID transponder
is configured to send a message to a reader, in particular an RFID reader or RFID
interrogator, during a transmission frame using active load modulation, ALM. The transponder
comprises a coding and modulation unit designed to generate a transmission signal
based on an encoded bit signal. The encoded bit signal has a first logic level during
first time segments within the transmission frame and a second logic level during
second time segments within the transmission frame. Therein, the first time segments
comprise an initial time segment of the transmission frame.
[0011] The transmission signal is generated having a first phase depending on the first
logic level during the first time segments, in particular during each of the first
time segments, and a second phase depending on the second logic level during the second
time segments, in particular during each of the second time segments. Furthermore,
the transmission signal is generated having the second phase during a time interval
preceding the transmission frame. The first and the second phase differ for example
by a predefined phase difference.
[0012] The initial time segment begins at the same time as the transmission frame. In particular,
a starting time of the initial time segment is equal to a starting time of the transmission
frame. Consequently, the transmission signal has the first phase directly after the
start of the transmission frame. The phase of the transmission signal changes from
the first to the second phase at the end of the initial time segment, in particular
at a starting time of one of the second time segments following, in particular directly
following, the initial time segment.
[0013] The sending of the message may begin with the transmission frame. In particular,
no part of the message is being transmitted during the time interval preceding the
transmission frame.
[0014] The time interval preceding the transmission frame ends when the transmission frame
starts, in particular an end time of the time interval preceding the transmission
frame is equal to the starting time of the transmission frame.
[0015] In some implementations, the time interval preceding the transmission frame starts
at a predefined starting time of the time interval preceding the transmission frame.
[0016] The starting time of the preceding time interval may for example be predefined with
respect to the starting time of the transmission frame. Then, the starting time of
the preceding time interval lies for example a predefined period of time before the
starting time of the transmission frame.
[0017] In some implementations, the transmission frame corresponds to a reply frame in response
to a request of the reader.
[0018] In some implementations, the starting time of the preceding time interval may for
example be predefined with respect to the request of the reader. Then, the starting
time of the preceding time interval lies for example a predefined period of time after
the request of the reader.
[0019] The reader antenna signal may have a first modulated value, in particular a first
modulated amplitude value, during the first time segments within the transmission
frame and a second modulated value, in particular a second modulated amplitude value,
during the second time segments within the transmission frame. A difference between
the first and the second modulated value of the reader antenna signal may origin from
the phase difference between the first and the second phase of the transmission signal.
Since the transmission signal is generated having the second phase during the time
interval preceding transmission frame, the reader antenna signal may have the second
modulated value also during the preceding time interval.
[0020] Consequently, considering the time interval preceding the transmission frame and
the first and the second time segments within the transmission frame, the reader antenna
signal change only between two different values, in particular amplitude values. Therefore,
an error rate of the communication between the RFID transponder and the reader, in
particular an error rate when demodulating the reader antenna signal, may be decreased
by means of the improved concept. At the same time, since the transmission signal
is generated during the first time segments as well as during the second time segments,
the ALM amplitude may be increased, leading for example to an increased operating
range.
[0021] Furthermore, a change, in particular an amplitude change, for example an absolute
value of the amplitude change, of the reader antenna signal at the starting time of
the transmission frame may be equal to a change of the reader antenna signal at a
transition between one of the first time segments and one of the second time segments
within the transmission frame, in particular at a transition from one of the second
time segments to one of the first time segments. Hence, a detection of a start of
the message by the reader may be improved and an error rate for detecting the start
of the message may be decreased.
[0022] According to some implementations, the coding and modulation unit is designed to
generate the transmission signal by modulating an oscillator signal with the encoded
bit signal, the oscillator signal.
[0023] According to some implementations, the transmission signal has an oscillator frequency
of the oscillator signal or approximately the oscillator frequency during the first
and the second time segments and during the time interval preceding the transmission
frame.
[0024] According to several implementations, the first logic level corresponds to logic
high and the second logic level corresponds to logic low or vice versa during the
transmission frame. In particular, the first logic level may correspond to logic high
during the transmission frame and to logic low during a further transmission frame
or vice versa. The same holds analogously for the second logic level.
[0025] According to several implementations, the first phase corresponds to a phase of the
oscillator signal or has a defined constant relation with respect to the phase of
the oscillator signal.
[0026] In some implementations, the predefined phase difference between the first and the
second phase is equal to 180° or approximately 180°.
[0027] According to several implementations, the predefined phase difference is generated
by means of an inverter circuitry of the RFID transponder, for example of the coding
and modulation unit.
[0028] According to several implementations, a value of the encoded bit signal is constant
or approximately constant during each of the first and the second time segments.
[0029] According to several implementations of the RFID transponder, the RFID transponder
is implemented as a near field communication, NFC, transponder.
[0030] According to several implementations, the oscillator frequency is equal to or approximately
equal to 13.56 MHz. Such transponders may be denoted as high-frequency, HF, RFID transponders.
[0031] According to several implementations, the coding and modulation unit is designed
to generate the encoded bit signal based on a data bit signal, wherein the data bit
signal represents data to be transmitted by the RFID transponder to the reader, in
particular represents the message.
[0032] According to several implementations, the RFID transponder comprises an antenna system
and a front end circuitry connected to the antenna system. The antenna system and
the front end circuitry are configured to generate a transmission radio frequency,
RF, field based on the transmission signal.
[0033] The transmission RF field may modulate the reader antenna signal for example by inducing
a signal change of the reader antenna signal, in particular by magnetic coupling.
In this way, the transmission RF field may be detected by the reader. Consequently,
data, in particular the message, may be transmitted from the RFID transponder to the
reader.
[0034] According to several implementations, the RFID transponder is configured to operate
in accordance with an industrial standard. In some implementations, the RFID transponder
is configured to operate in accordance with an industrial standard with respect to
generating the encoded bit signal based on the data bit signal. The industrial standard
may for example be a standard according to ISO/IEC 14443 Type A, ISO/IEC 14443 Type
B, JIS.X.6319-4 or another suitable standard.
[0035] According to several implementations, the coding and modulation unit is designed
to apply a predefined coding algorithm to the data bit signal for generating the encoded
bit signal.
[0036] According to some implementations, the encoded bit signal is given by the data bit
signal encoded by means of a Manchester coding algorithm. Such implementations may
for example correspond to implementations wherein the RFID transponder is configured
to operate in accordance with the JIS.X.6319-4 standard.
[0037] According to several implementations, the coding and modulation unit is designed
to generate the encoded bit signal based on the data bit signal and a subcarrier signal,
wherein the subcarrier signal is a binary clock signal with a subcarrier frequency
being smaller than the oscillator frequency. The subcarrier frequency may for example
be equal to or approximately equal to 847.5 MHz or 848 MHz.
[0038] According to several implementations, the encoded bit signal corresponds to the subcarrier
signal with a phase, in particular a code phase, depending on a logic level of the
data bit signal.
[0039] According to some implementations, the coding and modulation unit is designed to
generate the encoded bit signal by modulating the subcarrier signal depending on the
data bit signal according to phase shift keying, PSK, in particular according to binary
phase shift keying, BPSK. Such implementations may for example correspond to implementations
wherein the RFID transponder is configured to operate in accordance with the ISO/IEC
14443 Type B standard.
[0040] According to some implementations, a length of the time interval preceding the transmission
frame is greater than a specified minimum recovery time.
[0041] According to some implementations, the minimum recovery time corresponds to a time
required by the reader to recover from noise or an error.
[0042] According to some implementations, the minimum recovery time is specified by a transmission
protocol for the communication between the RFID transponder and the reader or between
a passive load modulation RFID transponder and the reader.
[0043] The minimum recovery time may for example be specified in the transmission protocol
for the sake of error reduction, in particular in passive load modulation systems.
Passive transponders may extract their power supply from a signal induced on their
antenna by a reader field. During data processing, which may start in a passive transponder
after a reader command has been received, current consumption in the passive transponder
may increase. The passive transponder supply current is for example supplied in pulses
synchronous to an internal clock used for data processing. A variation of passive
transponder current consumption may be misinterpreted as load modulation by the reader
and may therefore be misinterpreted as start of the passive transponder reply.
[0045] The EMVco contactless specification may for example require that a reader is able
to receive a frame at T
RECOVERY = 1280/(13.56 MHz) after a noise or corrupted frame has been received.
[0046] According to a second example, the minimum recovery time may be specified by the
International Standard ©ISO/IEC 14443-3. The minimum recovery time may for example
correspond to one of the low EMD times t
E,PICC or t
E,PCD as specified in Amendment 1 dated October 15, 2011 to the Second Edition of said
standard dated April 15, 2011.
[0047] The low EMD time may be a time before a start of a transponder reply frame or the
transmission frame during which a passive transponder may not be allowed to emit a
load modulation signal above a specified value. Further, the reader may have to be
able to receive the reply frame or transmission frame in case a frame with errors
was detected right before the low EMD time period.
[0048] The examples for the minimum recovery time are not to be considered limiting and
may be adapted to an actual implementation of the RFID transponder as recognized by
the skilled reader.
[0049] According to further implementations, the minimum recovery time may be specified
in any suitable way by another industrial or proprietary standard for communication
between the RFID transponder and the reader.
[0050] If the length of the preceding time interval is greater than the specified minimum
recovery time, it may be ensured that perturbations in the reader antenna signal due
to a start of generation of the transmission signal during the preceding time interval
do not influence the communication or the sending of the message during the transmission
frame, since the reader may have enough time to recover from the perturbations. In
particular, perturbations or errors when detecting the start of the message by the
reader may be avoided.
[0051] According to some implementations, the coding and modulation unit is designed to
ramp up an amplitude of the transmission signal from a reduced value to a full value
during a ramp-up interval within the interval preceding the transmission frame. A
length of the ramp-up interval corresponds for example to a time needed for ramping
up the amplitude from the reduced value to the full value.
[0052] The full value corresponds for example to an amplitude of the transmission signal
during the transmission frame, in particular during the second time segments, for
example during the first and the second time segments.
[0053] According to some implementations, the amplitude of the transmission signal reaches
the full value at or before the starting time of the transmission frame.
[0054] According to some implementations, the reduced value is equal to zero or equal to
a value between zero and the full value.
[0055] According to some implementations, the ramp-up interval is longer than a predefined
minimum period for transitions of the encoded bit signal between the first and the
second logic level.
[0056] Transitions of the encoded bit signal between the first and the second logic level
may occur at most with a specified maximum transition frequency. The predefined minimum
period is for example given by an inverse of the maximum transition frequency. The
maximum transition frequency may for example be given by the subcarrier frequency
or by a Manchester coding frequency.
[0057] The reader may contain filters, which may pass only signals within an expected band.
In case of RFID systems compliant to the ISO14443 standard, for example a 848kHz subcarrier
frequency may be used for the communication between transponder and reader for bit
rates of 106 kb/s, 212 kb/s, 424 kb/s and/or 848 kb/s. Thus, the reader may for example
reject frequency components below 848 kHz. If the transmission signal is ramped up
from the reduced to the full value for example within a time which is equal to one,
two or more than two periods of the subcarrier, the ramping may therefore not be recognized
by the reader.
[0058] Since the ramp-up interval is longer than the predefined minimum period, the reader
may not be disturbed, in particular may reject or may not recognize a change of the
reader antenna signal due to the ramping up of the transmission signal during the
ramp-up interval. Consequently, perturbations in the reader antenna signal due to
the start of generation of the transmission signal during the preceding time interval
may be avoided. In particular, perturbations or errors when detecting the start of
the message by the reader and/or when sending the message during the transmission
frame may be avoided.
[0059] According to some implementations, the first time segments comprise a final time
segment of the transmission frame and the transmission signal is generated having
the second phase during a time interval succeeding the transmission frame.
[0060] According to some implementations, the second time segments comprise the final time
segment of the transmission frame and the transmission signal is generated having
the first phase during the time interval succeeding the transmission frame.
[0061] The final time segment ends together with the transmission frame. In particular,
an end time of the final time segment is equal to an end time of the transmission
frame. Consequently, the phase of the transmission signal changes from the first phase
to the second phase or vice versa at the end of the transmission frame.
[0062] The sending of the message ends with the transmission frame. In particular, no part
of the message is being transmitted during the time interval succeeding the transmission
frame.
[0063] The time interval succeeding the transmission frame starts when the transmission
frame ends. In particular, a starting time of the time interval succeeding the transmission
frame is equal to the end time of the transmission frame.
[0064] If the transmission signal is generated with its phase changing from the first phase
to the second phase or vice versa at the end of the transmission frame, also the reader
antenna signal may correspondingly change from the first modulated value to the second
modulated value, or vice versa, at the end of the transmission frame.
[0065] Consequently, considering the time intervals preceding and succeeding the transmission
frame and the first and the second time segments within the transmission frame, the
reader antenna signal may have only two different values, in particular amplitude
values. Therefore, an error rate of the communication between the RFID transponder
and the reader, in particular an error rate when demodulating the reader antenna signal,
may be further decreased. Furthermore, a change, in particular an amplitude change,
for example an absolute value of the amplitude change, of the reader antenna signal
at the end time of the transmission frame may be equal to the change of the reader
antenna signal at a transition between one of the first time segments and one of the
second time segments within the transmission frame. Hence, a detection of an end of
the message by the reader may be improved and an error rate for detecting the end
of the message may be decreased.
[0066] According to some implementations, a length of the time interval succeeding the transmission
frame is greater than the specified minimum recovery time. It is referred to the explanations
regarding the length of the preceding time interval in this respect.
[0067] According to some implementations, the coding and modulation unit is designed to
ramp down the amplitude of the transmission signal from the full value to a first
further reduced value during a ramp-down interval within the time interval succeeding
the transmission frame. A length of the ramp-down interval corresponds for example
to a time needed for ramping down the amplitude from the full value to the first further
reduced value.
[0068] According to some implementations, the first further reduced value is equal to zero
or equal to a value between zero and the full value.
[0069] According to some implementations, the ramp-down interval is longer than the predefined
minimum period for transitions of the encoded bit signal between the first and the
second logic level.
[0070] According to some implementations, the transmission frame comprises a further time
segment in addition to the first and the second time segments. The encoded bit signal
may have the first or the second logic level during the further time segment.
[0071] According to some implementations, the transmission signal, in particular the generation
of the transmission signal, is paused at least during a part of the further time segment.
[0072] According to some implementations, the further time segment corresponds to a silence
period according to a Manchester code.
[0073] According to some implementations, the encoded bit signal has the second logic level
during the further time segment and the coding and modulation unit is designed to
generate the transmission signal having the second phase at least during a part of
the further time segment.
[0074] According to some implementations, the encoded bit signal has the first logic level
during the further time segment and the coding and modulation unit is designed to
generate the transmission signal having the first phase at least during a part of
the further time segment.
[0075] According to some implementations, the transmission signal is generated continuously
during the further time segment.
[0076] According to some implementations, the coding and modulation unit is designed to
ramp up the transmission signal during a final portion of the further time segment
from a second further reduced value to the full value. The second further reduced
may be zero or a value between zero and the full value.
[0077] In some implementations, the coding and modulation unit is designed to ramp down
the transmission signal during an initial portion of further time segment from the
full value to the second or to a third further reduced value. The third further reduced
may be zero or a value between zero and the full value.
[0078] In some implementations the transmission frame consists of the first and the second
time segments.
[0079] According to the improved concept, also a method for sending a message from the RFID
transponder to a reader, in particular an RFID reader, during a transmission frame
using ALM is provided. The method comprises generating a transmission signal based
on an encoded bit signal. The encoded bit signal has a first logic level during first
time segments within the transmission frame and a second logic level during second
time segments within the transmission frame. Therein, the first time segments comprise
an initial time segment of the transmission frame.
[0080] The transmission signal is generated having a first phase depending on the first
logic level during the first time segments and a second phase depending on the second
logic level during the second time segments. Furthermore, the transmission signal
is generated having the second phase during a time interval preceding the transmission
frame.
[0081] Further implementations of the method are readily derived from the various implementations
of the RFID transponder and vice versa.
[0082] For further details regarding the method, it is referred to the explanations with
respect to the RFID transponder according to the improved concept.
[0083] In the following, the invention is explained in detail with the aid of exemplary
implementations by reference to the drawings. Components that are functionally identical
or have an identical effect may be denoted by identical references.
Identical components and/or components with identical effects may be described only
with respect to the figure where they occur first and their description is not necessarily
repeated in subsequent figures.
[0084] In the drawings,
Figure 1 shows an exemplary implementation of an RFID transponder according to the
improved concept;
Figure 2A shows signal sequences as a function of time occurring in an exemplary implementation
of an RFID transponder according to the improved concept and a reader;
Figure 2B shows signal sequences as a function of time occurring in a further exemplary
implementation of an RFID transponder according to the improved concept and a reader;
Figure 3 shows signal sequences as a function of time occurring in a further exemplary
implementation of an RFID transponder according to the improved concept and a reader;
Figure 4A shows signal sequences as a function of time occurring in a further exemplary
implementation of an RFID transponder according to the improved concept and a reader;
and
Figure 4B shows signal sequences as a function of time occurring in a further exemplary
implementation of an RFID transponder according to the improved concept and a reader.
[0085] Figure 1 shows an exemplary implementation of an RFID transponder according to the
improved concept. The RFID transponder comprises a front end circuitry FE and an antenna
system A connected to the front end circuitry FE. The RFID transponder further comprises
a clock circuit CLK connected to the front end circuitry FE and a coding and modulation
unit CMU connected between the clock circuit CLK and the front end circuitry FE.
[0086] The antenna system A may for example be configured to detect a radio-frequency, RF,
field, for example generated by a reader (not shown), in particular an RFID reader,
communicating with the RFID transponder. The antenna system A and the front end circuitry
FE may for example generate a reconstructed reader signal SR based on the detected
RF field. The reconstructed reader signal SR may be supplied to the clock circuit
CLK.
[0087] The clock circuit CLK may generate an oscillator signal SO for example based on,
in particular temporarily based on, the reconstructed reader signal SR, the oscillator
signal SO having an oscillator frequency. For example, the oscillator frequency may
be given by or approximately given by 13.56 MHz. The clock circuit CLK supplies the
oscillator signal SO for example to the coding and modulation unit CMU. Furthermore,
the coding and modulation unit CMU may receive a data bit signal SD for example from
a further component (not shown) of the RFID transponder. The data bit signal SD represents
for example data, in particular a message, to be transmitted by the RFID transponder
to the reader. Based on the data bit signal SD, the coding and modulation unit CMU
may generate for example an encoded bit signal SE.
[0088] For generating the encoded bit signal SE, the coding and modulation unit CMU may
for example apply a predefined coding algorithm, for example a Manchester coding algorithm,
to the data bit signal SD. Alternatively or in addition, the coding and modulation
unit CMU may generate the encoded bit signal SE based on the data bit signal SD and
a subcarrier signal. The subcarrier signal may for example be a binary clock signal
with a subcarrier frequency being smaller than the oscillator frequency.
[0089] The coding and modulation unit CMU is further configured to generate a transmission
signal ST based on the encoded bit signal SE, in particular by modulating the oscillator
signal SO with the encoded bit signal SE.
[0090] The coding and modulation unit CMU may supply the transmission signal ST to the front
end circuitry FE. Based on the transmission signal ST, the front end circuitry FE
and the antenna system A may generate a transmission RF field. A reader antenna signal
SRA on an antenna of the reader may be modulated according to the transmission RF
field. In this way, the message may for example be sent from the RFID transponder
to the reader.
[0091] A requirement of ALM may be that a phase difference between a signal induced by the
reader and a signal generated by the RFID transponder is for example constant or approximately
constant during each transmission frame during which a message may be sent from the
RFID transponder to the reader. In some implementations according to the improved
concept, the oscillator signal SO is for example in phase with the reconstructed reader
signal SR continuously during each transmission frame. In particular, a synchronization
of the clock signal SO with the reconstructed reader signal SR may for example not
take place within a transmission frame.
[0092] For further details regarding the operation of the RFID transponder, in particular
on the generation of the transmission signal ST by the coding and modulation unit
CMU, it is referred to Figures 2A through 4B.
[0093] Figure 2A shows signal sequences as a function of time occurring in an exemplary
implementation of an RFID transponder according to the improved concept, for example
an RFID transponder as shown in Figure 1, and a reader. In particular, the encoded
bit signal SE, the transmission signal ST and the reader antenna signal SRA are shown
schematically as a function of time t.
[0094] A message is to be sent from the RFID transponder to the reader during a transmission
frame. The transmission frame starts at a starting time TFS. An end time of the transmission
frame is not shown in Figure 2A. Before the starting time TFS, the encoded bit signal
SE is equal to zero. Within the transmission frame, that is after the starting time
TFS, the encoded bit signal SE has a first logic level, for example logic high, during
first time segments including an initial time segment S1. A starting time of the initial
time segment S1 is equal to the starting time TFS of the transmission frame. During
second time segments, including a time segment S2 following directly after the initial
time segment S1, the encoded bit signal SE has a second logic level, for example logic
low, being different from the first logic level.
[0095] The coding and modulation unit CMU may generate the transmission signal ST by modulating
the oscillator signal SO with the encoded bit signal SE. In particular, during the
first time segments, the transmission signal ST may comprise signal pulses (not shown)
with a frequency given by the oscillator frequency and with a first phase. With respect
to a phase of the oscillator signal SO, the first phase may for example be zero or
approximately zero or given by a predefined constant value. During the second time
segments, the transmission signal ST may comprise signal pulses (not shown) with a
frequency given by the oscillator frequency and with a second phase. The second phase
may have a predefined value with respect to the first phase, for example 180°. In
Figure 2A, blank segments of the transmission signal ST depict times where the transmission
signal ST has the first phase, for example 0°, while shaded segments depict times
where the transmission signal ST has the second phase, for example 180°.
[0096] During a time interval PI preceding the transmission frame, the coding and modulation
unit CMU may generate the transmission signal ST having the second phase, in particular
may generate the transmission signal ST the same way as during the second time segments.
It is highlighted that the transmission frame has not begun yet during the preceding
time interval PI and no message is sent from the RFID transponder to the reader during
the preceding time interval PI.
[0097] In the lowest panel of Figure 2A, a response of the reader antenna signal SRA to
the transmission signal ST, in particular to the transmission RF field generated by
the front end circuitry FE and the antenna system A based on the transmission signal
ST, is shown schematically. In particular, only an amplitude value of the reader antenna
signal SRA is shown, while individual oscillations of the antenna reader signal SRA
are not shown for the sake of clarity. The oscillations of the antenna reader signal
may for example have a frequency given by or approximately given by the oscillator
frequency.
[0098] During times when the transmission signal ST is not generated, for example before
the preceding time interval PI, the amplitude of the reader antenna signal SRA has
an unmodulated value V0. Whenever the transmission signal ST is generated having the
first phase, in particular during the first time segments, the amplitude of the reader
antenna signal SRA has a first modulated value V1, which may be greater or less than
the unmodulated value V0. In the example of Figure 2A, the first modulated value V
is less than the unmodulated value V0. Whenever the transmission signal ST is generated
having the second phase, in particular during the second time segments and during
the preceding time interval PI, the amplitude of the reader antenna signal SRA has
a second modulated value V2, which may be different from the first modulated value
V1 and may be less or greater than the unmodulated value V0. In the example of Figure
2A, the second modulated value V2 is greater than the unmodulated value V0.
[0099] Is highlighted, that the exact differences between the unmodulated value V0 and the
first and the second modulated values V1, V2, respectively, depend on the values of
the first and the second phases, respectively. In implementations, where the phase
difference between the first and the second phase is 180°, a difference between the
first modulated value V1 and the unmodulated value V0 may have the same absolute value
as a difference between the second modulated value V2 and the unmodulated value V0.
[0100] Importantly, since the transmission signal ST has the second phase during the preceding
time interval PI, the reader antenna signal SRA has the same amplitude value during
the preceding interval PI as during the second time segments. Consequently, considering
the first time segments, the second time segments and the preceding time interval
PI, the reader antenna signal SRA has only two different amplitude values, namely
the first and the second modulated value V1, V2. Therefore, an error rate of the communication
between the RFID transponder and the reader, in particular an error rate when demodulating
the reader antenna signal, may be decreased.
[0101] Furthermore, an absolute change of the amplitude value of the reader antenna signal
SRA is the same at the starting time TFS of the transmission frame and at transitions
between the first and the second time segments within the transmission frame. Hence,
error rate for detecting a start of the message may be decreased.
[0102] The length of the preceding time interval PI may for example be longer than a minimum
recovery time TR. The minimum recovery time TR may for example correspond to a time
required by the reader to recover from noise or an error. It may be specified for
example by a proprietary or non-proprietary industrial standard.
[0103] Consequently, it may be avoided that perturbations in the reader antenna signal due
to a start of generation of the transmission signal ST at the beginning of the preceding
time interval influence the sending of the message during the transmission frame or
detection of the start of the message. This may be especially beneficial for example
in case of an ISO14443 Type A 106 kb/s communication protocol, where a transponder
reply may be expected in a narrow time slot and there may be no training sequence
in the beginning of the message.
[0104] Figure 2B shows signal sequences as a function of time occurring in a further exemplary
implementation of an RFID transponder according to the improved concept, for example
an RFID transponder as shown in Figure 1, and a reader.
[0105] In the example of Figure 2B, the encoded bit signal SE is for example logic low during
the first time segments and logic high during the second time segments. Consequently,
the values of the first and the second phase may be switched compared to the example
of Figure 2A. For example, the second phase may be zero or approximately zero with
respect to the oscillator signal SO, while the first phase may have a predefined value
with respect to the second phase, for example 180°. In contrast to Figure 2A, blank
segments of the transmission signal ST depict times where the transmission signal
ST has the second phase, for example 0°, while the shaded segments depict times where
the transmission signal ST has the first phase, for example 180°, in Figure 2B.
[0106] It follows that, referring to the explanations with respect to Figure 2A, the first
modulated value V1 of the reader antenna signal SRA may be greater than the unmodulated
value V0, while the second modulated value V2 may be less than the unmodulated value
V0.
[0107] Apart from these differences, the explanations regarding Figure 2A hold analogously
for Figure 2B.
[0108] In particular, the examples of Figures 2A and 2B may correspond to the same exemplary
implementation of an RFID transponder according to the improved concept, wherein the
transmission frames of Figures 2A and 2B are different transmission frames.
[0109] Figure 3 shows signal sequences as a function of time occurring in a further exemplary
implementation of an RFID transponder according to the improved concept, for example
an RFID transponder as shown in Figure 1, and a reader. Implementation of Figure 3
is for example based on the implementation explained with respect to Figure 2A.
[0110] In the example of Figure 3, the length of the preceding time interval PI is not necessarily
longer than the minimum recovery time TR.
[0111] The transmission signal ST is ramped up for example from a reduced value, for example
from zero, to its full value during a ramp-up interval RU within the preceding time
interval PI, in particular at the beginning of the preceding time interval PI. The
full value is for example equal to a value of the transmission signal ST during the
second time segments. Consequently, the amplitude of the reader amplitude signal SRA
ramps from the unmodulated value V0 to the second modulated value V2 during the ramp-up
interval RU.
[0112] The ramp-up interval RU is for example longer than a predefined minimum period for
transitions of the encoded bit signal SE between the first and the second logic level.
The minimum period for transitions between the first and the second logic level is
for example a specified minimum time for the encoded bit signal SE to change from
the first logic level to the second logic level back to the first logic level and
again to the second logic level. The specified minimum time may for example be given
by an inverse of the subcarrier frequency.
[0113] Since the ramp-up interval RU is for example longer than the predefined minimum period,
the change of the reader antenna signal SRA during the ramp-up interval RU may not
be detected, for example may be filtered out or may be rejected, by the reader. Consequently,
perturbations in the reader antenna signal SRA due to the start of the generation
of the transmission signal ST may be avoided.
[0114] Figure 4A shows signal sequences as a function of time occurring in a further exemplary
implementation of an RFID transponder according to the improved concept, for example
an RFID transponder as shown in Figure 1, and a reader.
[0115] According to Figure 4A, a transmission frame ends at an end time TFE. The transmission
frame of Figure 4A may for example correspond to the transmission frame of one of
Figures 2A, 2B or 3. The starting time TFS of the transmission frame is not shown
in Figure 4A. In the following, it is assumed for explanatory reasons only that the
transmission frame of Figure 4A corresponds to the transmission frame of Figure 2A
or Figure 3. However, the explanations are readily adapted to the example of Figure
2B.
[0116] After the end time TFE, the encoded bit signal SE is for example equal to zero. In
Figure 4A, the second time segments include a final time segment S3 of the transmission
frame. An end time of the final time segment S3 is equal to the end time TFE of the
transmission frame. The first time segments include a time segment S4 preceding the
final time segment S3. Thus, the encoded bit signal SE has the second logic level
during the final time segment S3 and the first logic level during the time segment
S4.
[0117] During a time interval SI succeeding the transmission frame, the coding and modulation
unit CMU may generate the transmission signal ST having the first phase, in particular
generate the transmission signal ST the same way as during the first time segments.
It is highlighted, that the transmission frame has already ended during the succeeding
time interval SI and no message is sent from the RFID transponder to the reader during
the succeeding time interval SI.
[0118] Since the transmission signal ST has the first phase during the succeeding time interval
SI, the reader antenna signal SRA has the same amplitude during the succeeding interval
SI as during the first time segments, for example the first modulated value V1. Consequently,
considering the first time segments, the second time segments and the succeeding time
interval SI, the reader antenna signal SRA has only two different amplitude values,
namely the first and the second modulated value V1, V2. Therefore, an error rate of
the communication between the RFID transponder and the reader, in particular an error
rate when demodulating the reader antenna signal SRA, may be decreased.
[0119] Furthermore, an absolute change of the amplitude value of the reader antenna signal
SRA is the same at the end time TFE of the transmission frame and at transitions between
the first and the second time segments within the transmission frame. Hence, error
rate for detecting an end of the message may be decreased.
[0120] Figure 4B shows signal sequences as a function of time occurring in a further exemplary
implementation of an RFID transponder according to the improved concept, for example
an RFID transponder as shown in Figure 1, and a reader.
[0121] According to Figure 4B, a transmission frame ends at an end time TFE. The transmission
frame of Figure 4A may for example correspond to the transmission frame of one of
Figures 2A, 2B or 3. The starting time TFS of the transmission frame is not shown
in Figure 4B. In the following, it is assumed for explanatory reasons only that the
transmission frame of Figure 4B corresponds to the transmission frame of Figure 2A
or Figure 3. However, the explanations are readily adapted to the example of Figure
2B.
[0122] After the end time TFE, the encoded bit signal SE is for example equal to zero. In
Figure 4B, the first time segments include a final time segment S3 of the transmission
frame. An end time of the final time segment S3 is equal to the end time TFE of the
transmission frame. The second time segments include a time segment S4 preceding the
final time segment S3. Thus, the encoded bit signal SE has the first logic level during
the final time segment S3 and the second logic level during the time segment S4.
[0123] During a time interval SI succeeding the transmission frame, the coding and modulation
unit CMU may generate the transmission signal ST having the second phase, in particular
generate the transmission signal ST the same way as during the second time segments.
[0124] Since the transmission signal ST has the second phase during the succeeding time
interval SI, the reader antenna signal SRA has the same amplitude during the succeeding
interval SI as during the second time segments, for example the second modulated value
V2.
[0125] In Figures 4A and/or 4B, the length of the succeeding time interval SI may for example
be longer than the minimum recovery time TR.
[0126] Alternatively or in addition, the transmission signal ST may be ramped down during
a ramp-down interval (not shown) within the succeeding interval SI, in particular
at an end of the succeeding interval SI. In this respect, it is referred to the explanations
regarding the ramping up according to Figure 3, which hold analogously for the ramping
down.
[0127] It is highlighted that, while in Figures 2A through 4B for example the amplitude
of the antenna reader signal SRA is modulated due to the transmission signal ST, also
a phase modulation or an amplitude and phase modulation may be achieved depending
on the relations of the first and the second phase with respect to each other and/or
with respect to the phase of the antenna reader signal SRA.
[0128] It is further pointed out that the drawings, in particular the differences between
the unmodulated value V0 and the first and the second modulated value V1, V2 of the
antenna reader signal SRA, are not necessarily drawn to scale.
[0129] By means of the improved concept, a high load modulation amplitude may be achieved
without the drawbacks of existing approaches, such as reduced success rate of transponder
reply or increased error rates during communication and when detecting the start of
a message. In particular, due to the described generation of the transmission signal
ST during the preceding time interval PI for example in combination with the generation
of the transmission signal ST during the first as well as during the second time segments,
said drawbacks may be overcome.
[0130] Furthermore, by using an RFID transponder or method according to the improved concept,
there may be no need for an additional charge pump in the transponder to increase
an overall signal amplitude, which could otherwise be necessary due to a too low load
modulation amplitude.
Reference numerals
[0131]
- A
- antenna of transponder
- FE
- front end circuitry
- CLK
- clock circuit
- CMU
- coding and modulation unit
- SD
- data bit signal
- SE
- encoded bit signal
- ST
- transmission signal
- SO
- oscillator signal
- SR
- reconstructed reader signal
- SRA
- reader antenna signal
- S1
- initial time segment
- S3
- final time segment
- S2, S4
- time segments
- TFS
- starting time of transmission frame
- TFE
- end time of transmission frame
- TR
- minimum recovery time
- PI
- preceding time interval
- SI
- succeeding time interval
- V0
- unmodulated value
- V1, V2
- modulated values
- RU
- ramp-up interval
1. RFID transponder configured to send a message to a reader during a transmission frame
using active load modulation, the transponder comprising a coding and modulation unit
(CMU) designed to generate a transmission signal (ST) based on an encoded bit signal
(SE), wherein
- the encoded bit signal (SE) has a first logic level during first time segments within
the transmission frame and a second logic level during second time segments within
the transmission frame, wherein the first time segments comprise an initial time segment
of the transmission frame;
- the transmission signal (ST) is generated having
- a first phase depending on the first logic level during the first time segments;
- a second phase depending on the second logic level during the second time segments;
and
- the second phase during a time interval preceding the transmission frame (PI).
2. RFID transponder according to claim 1, wherein a length of the time interval preceding
the transmission frame (PI) is greater than a specified minimum recovery time (TR).
3. RFID transponder according to claim 2, wherein the minimum recovery time (TR) corresponds
to a time required by the reader to recover from noise or an error.
4. RFID transponder according to one of claims 2 or 3, wherein the minimum recovery time
(TR) is specified by a transmission protocol for a communication between the RFID
transponder and the reader.
5. RFID transponder according to one of claims 1 to 4, wherein the coding and modulation
unit (CMU) is designed to ramp up an amplitude of the transmission signal (ST) from
a reduced value to a full value during a ramp-up interval within the time interval
preceding the transmission frame (PI).
6. RFID transponder according to claim 5, wherein the ramp-up interval is longer than
a predefined minimum period for transitions of the encoded bit signal between the
first and the second logic level.
7. RFID transponder according to one of claims 1 to 6, wherein the transmission signal
(ST) is generated having, during a time interval succeeding the transmission frame
(SI),
- the second phase if the first time segments comprise a final time segment of the
transmission frame; and
- the first phase if the second time segments comprise the final time segment of the
transmission frame.
8. RFID transponder according to claim 7, wherein a length of the time interval succeeding
the transmission frame (SI) is greater than a specified minimum recovery time (TR).
9. RFID transponder according to one of claims 7 or 8, wherein the coding and modulation
unit (CMU) is designed to ramp down an amplitude of the transmission signal (ST) from
a full value to a further reduced value during a ramp-down interval within the time
interval succeeding the transmission frame (SI).
10. RFID transponder according to one of claims 1 to 9, wherein
- the encoded bit signal has the second logic level during a further time segment
of the transmission frame and the coding and modulation unit (CMU) is designed to
generate the transmission signal (ST) having the second phase at least during a part
of the further time segment; or
- the encoded bit signal has the first logic level during a further time segment and
the coding and modulation unit (CMU) is designed to generate the transmission signal
(ST) having the first phase at least during a part of a further time segment.
11. RFID transponder according to one of claims 1 to 10, wherein the coding and modulation
unit (CMU) is designed to generate the transmission signal (ST) by modulating an oscillator
signal with the encoded bit signal (SE).
12. RFID transponder according to one of claims 1 to 11, wherein the coding and modulation
unit (CMU) is designed to generate the encoded bit signal (SE) based on a data bit
signal (SD), wherein the data bit signal (SD) represents the message.
13. Method for sending a message from an RFID transponder to a reader during a transmission
frame using active load modulation, wherein the method comprises generating a transmission
signal (ST) based on an encoded bit signal (SE), wherein
- the encoded bit signal (SE) has a first logic level during first time segments within
the transmission frame and a second logic level during second time segments within
the transmission frame, wherein the first time segments comprise an initial time segment
of the transmission frame; and
- the transmission signal (ST) is generated having
- a first phase depending on the first logic level during the first time segments;
- a second phase depending on the second logic level during the second time segments;
and
- the second phase during a time interval preceding the transmission frame (PI).
14. Method according to claim 13, wherein a length of the time interval preceding the
transmission frame (PI) is greater than a specified minimum recovery time (TR).
15. Method according to one of claims 13 or 14, further comprising ramping up an amplitude
of the transmission signal (ST) from a reduced value to a full value during a ramp-up
interval within the time interval preceding the transmission frame (PI).